The majority of the world's telecommunications networks rely on proven digital communication technologies based on Time Division Multiplexing (TDM) and Pulse Code Modulation (PCM). Time Division Multiplexing is able to carry multiple data and digitized voice (normally PCM) channels over a variety of physical transmission media. Examples of such media are copper wire, optical fiber, and radio. Various TDM data speeds are used in communications, depending on the capabilities of the physical media or carrier. For example, the T-1 standard multiplexes twenty-four communications channels to make economical use of lower speed media (e.g. copper wires), while the T-3 standard multiplexes 672 communications channels for use on higher speed media (e.g. optical fiber).
TDM combines data samples from several different communications channels (for example, telephone calls) into a single aggregate synchronous bit pattern at a higher data speed. By so doing, several communications channels may economically share the same communications medium, whether it be a pair of copper wires, a strand of optical fiber, or a modulated radio frequency. Thus, TDM reduces the number of physical transmission resources needed in comparison to the alternative of dedicating a separate physical medium for each communications channel. For example, T-1 uses two copper wire pairs to carry twenty-four telephone channels, instead of using twenty-four individual copper wire pairs.
In addition to economical media use, digital Pulse Code Modulation and Time Division Multiplexing also provide very high-quality voice transmission over long distances. Analog modulations (for example, the output of a telephone set) are subject to reductions in volume level, frequency distortion, and injection of noise signals as the distance of transmission increases. Because TDM is a digital technique, the communications channels may be faithfully recreated (decoded) at their destination point, regardless of the distance traveled.
Because all TDM communications channels are encoded as digital data, TDM also lends itself to the digital switching of channels to multiple destinations (for example, telephone call routing). In summary, TDM provides economical media utilization, quality of voice transmission, ease of channel routing or switching, and the ability to carry a variety of voice and data traffic types over a common communications network. Because of economy and quality, TDM techniques provide the majority of the world's communications infrastructure and continue to grow daily in implementation.
An example of Time Division Multiplexing for the creation of a T-1 carrier aggregate output is shown in FIG. 1. In the T-1 Multiplexing example, each of the eight-bit digital data or voice samples has an effective data rate of 64 kilo bits per second. This data rate is derived from the international standard of eight-bit Pulse Code Modulation that samples voice at a rate of 8 kilo Hertz. Eight bits times 8 kilo Hertz results in 64 kilo bits per second of information at each of twenty-four input communications channels. The T-1 Time Division Multiplexer device creates a higher bit rate aggregate output from the twenty-four input channels. The data rate of the aggregate output is twenty-four channels times 64 kilo bits each of 1.536 kilo bits per second. The insertion of the Frame Bit by the T-1 Multiplexer accounts for an extra 8 kilo bits per second, to make the total transmission rate of T-1 aggregate 1,544 kilo bits per second. The Frame Bit allows the far-end T-1 demultiplexer to recognize the beginning order of the communications channels, and decode them in the proper sequence.
By placing each channel's data in a time-sequenced order on the line, it is easy for the far-end receiving demultiplexer to recreate the channels faithfully in the correct order. Time sequencing and synchronization of data speeds from end-to-end are fundamental properties of Time Division Multiplexing.
A basic need of most communications networks is the ability to mix communications channels from different TDM sources. FIG. 2 shows a technique called T-1 Drop and Insert Grooming, which is performed by many T-1 multiplexing devices available on the commercial market. Twenty-four communications channels are transmitted from Location X to Location Y over a T-1 carrier. Channels 13 through 24 that are used for communications between X and Y are groomed from the T-1 carrier at Location Y, where they would typically be decoded as twelve voice or data channels. The remainder of Location X's transmitted T-1 channels (1 through 12) are sent on by Location Y to Location Z. The channels marked by an X in the diagram are unusable (idle coded) channels in the T-1 carrier frame. Location X can communicate with Location Y via channels 13 through 24. Location X can communicate with Location Z via channels 1 through 12. Location Y cannot communicate with Location Z. This is a fundamental limitation of the utility of T-1 Drop and Insert Grooming.
A technique of combining channels from two (or many) TDM sources at Location Y is shown in FIG. 3. This technique of Time and Space Switching may include Time Slot Interchange (TSI), and forms the basis for both Digital Cross Connection and Switching (or call routing) devices. The Time and Space Switch can be configured to provide the less complex function of T-1 Drop and Insert Grooming, as previously explained. In addition to this function, it provides capabilities that address a much wider range of communications needs.
In the example shown in FIG. 3, channels that originate from the Location X T-1 carrier may be assigned by the Time and Space Switch for communications with Location Y. This is shown in this example as Data C in time slot 24 of the X T-1 being output in time slot 3 of the Y T-1.
Unlike the T-1 Drop and Insert Grooming example shown in FIG. 2, Location Y may communicate with Location Z through channels assigned by the Time and Space Switch at Location Y. An example is Data B in time slot 22 of the Y T-1 being output in time slot 1 of the Z T-1.
Location X may also be assigned to communicate with Location Z. An example is Data A in time slot 3 of the X T-1 being output in time slot 23 of the Z T-1.
Thus with a Time and Space Switch at Location Y, any of the three locations can be assigned communications channels with any other location. There are no restrictions, as with the Drop and Insert Grooming technique. By adding more T-1 connections to the Time and Space switch, more Locations can be added to the communications hub at Location Y.
The Drop and Insert Grooming technique fixes the time slot sequence of output channels to the same time position as the input channels. Channel 4 data from Location X will only appear in the Channel 4 position at Location Z, for example, regardless of particular channel assignment needs at the Z location.
With the Time and Space Switch replacing the Drop and Insert technique, Channel 4 data from Location X may be freely assigned to appear in any desired channel at location Z. Flexible assignment of channels provides users with more effective utilization of equipment and more easily managed resources at all locations.
A Digital Cross Connect function assigns channels between T-1 connections statically. The channel assignments do not typically change once they are programmed in the Time and Space Switch. In contrast, the Switching function makes use of the Time and Space Switch to dynamically change channel assignments on a call-by-call basis. Originating T-1 channels are connected to destination T-1 channels as telephone or data calls are routed between locations by the Switch. When a call is released, the channels are disconnected in the Time and Space Switch and made available for another call. Time and Space Switching is a fundamental function of large-scale transmission resource management devices (Digital Cross Connect Systems) and Digital Telecommunications Switches in the public communications network, world-wide.
Of course, there are already many network access devices based on TDM and PCM, including hardware implementations such as conventional single T-1 digital channel banks and broadband T-1 access multiplexers. But such equipment is generally large, expensive, and relatively inflexible in its functionality.
Complex hardware designs including TSI circuitry suffer from the disadvantages of an undesirable amount of circuit board "real estate" being used as well as an undesirable amount of power consumption. Perhaps even more important is the amount of frame delay inherent in most TSI implementations, with delays in the range of two to three frames. In addition, most hardware TSI designs are inherently inflexible in their application and potential for modification, as well as upward scalability.
It is against this background and the desire to solve the problems of the prior art that the present invention has been developed.